High tensile strength steel plates with a tensile strength of 780 MPa or more which are used as welding structural members for construction machines, industrial machines, bridges, buildings, ships and the like are required to have, in addition to compatibility between high strength and high toughness of a base material, high preheating-free high weldability and excellent low-temperature toughness of a welded joint with an increase in the need for constructional members with a high strength and an increase in use in cold regions. In addition, thick steel plates of 780 MPa or more which satisfy all such features and can be manufactured at low cost in a short construction time are required to have a thickness of up to about 40 mm. Therefore, steel plates are required to satisfy all three features, (a) high strength and high toughness of a base material, (b) a preheating-free characteristic in low heat input welding where the heat input amount is 2.0 kJ/mm or less, and (c) low-temperature toughness of a welded joint, with a low-cost component system in a short construction time and low cost manufacturing process.
As a conventional method of manufacturing high tensile strength thick steel plates of 780 MPa or more which have high weldability applied thereto, for example, Patent Documents 1 to 3 disclose a method with direct hardening and tempering, including processes of directly hardening a steel plate in an on-line process immediately after the steel plate is rolled, and subsequently tempering the steel plate.
Regarding methods of manufacturing high tensile strength thick steel plates of 780 MPa or more involving no thermal refining, for example, Patent Documents 4 to 8 disclose manufacturing methods which are excellent in terms of manufacturing time period and productivity from the viewpoint that a reheating tempering heat treatment can be omitted. Among these Patent Documents, Patent Documents 4 to 7 disclose manufacturing methods which use an accelerated cooling mid-course stoppage process in which accelerated cooling after rolling of a steel plate is stopped in mid-course, and Patent Document 8 discloses a manufacturing method in which air cooling is performed after rolling to cool the temperature down to room temperature.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H03-232923    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H09-263828    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2000-160281    Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2000-319726    Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2005-15859    Patent Document 6: Japanese Unexamined Patent Application, First Publication No. 2004-52063    Patent Document 7: Japanese Unexamined Patent Application, First Publication No. 2001-226740    Patent Document 8: Japanese Unexamined Patent Application, First Publication No. H08-188823